Actually you don't "need" a mill but if you knew how to run one well enough to do the work it would sure do a cleaner and faster job.
I did an HTG that had no mag well cut, no recoil lug cut and no trigger guard cut. Was it easy to make it into a BDL? It wasn't as difficult as I thought and was afraid I would make a mistake as it was molded in color in white,gray and tan. Came out great but if I had my drothers I would sure as hell have ordered it that way. This was a purchase from another member and a "I can get it right away for my coyote rifle" brain fart. Made a real nice 6x47L I will have to say.
Just depends on how handy you are and if you can see what it should look like when done yet <span style="font-weight: bold">BEFORE </span> you do it.